The present invention pertains to a process for the production of pellets containing insoluble sulfur and the pellets thus produced.
Insoluble sulfur compositions, which include substantial proportions of polymeric sulfur which is an amorphous form of sulfur that is insoluble in carbon disulfide are vulcanizing agents for rubber and in many cases are used for this purpose in preference to elemental sulfur, which are also known as rubbermaker's sulfur, that is, alpha-rhombic and beta-monoclinic crystals of sulfur, both of which are completely soluble in carbon disulfide. The solubility of elemental sulfur in rubber increases with increasing temperature. Sulfur crystallizes out on the surface as well as throughout a mixture of elemental sulfur and rubber that is mixed at elevated temperatures and cooled so that, because of this nonhomogeneous distribution of the sulfur, the rubber is likely to be overvulcanized or undervulcanized by the following vulcanization. Such crystallization of elemental sulfur is called "bloom".
The use of insoluble sulfur eliminates blooming and other difficulties, provided that it is distributed homogeneously throughout the rubber. To achieve this result the insoluble sulfur must be added in the form of very small particles. Such fine powders, however, can be handled only with difficulty. Not only does the incidental formation of dust result in loss of material, but it is also often a great nuisance to workmen. Furthermore, elimination of dust from a plant requires costly equipment. The poor flowability of insoluble sulfur impedes its transportation, storage, dispensing from storage bins, as well as its metering. Furthermore, particles of insoluble sulfur have the property of becoming strongly charged electrostatically by rubbing against each other, and thereby increase the possibility of the occurrence of an explosion. The homogeneous distribution and thermal stability of commercial insoluble sulfur compositions in rubber mixtures increases with increasing purity as well as crystallinity of the compositions. Insoluble sulfur compositions which contain more than 30% by weight of sulfur that is insoluble in carbon disulfide can generally be used in vulcanizing rubber. Because of the continually increasing use of fast-running high-efficiency mixers, in which sudden rises in temperatures up to 150.degree. C. can occur, sulfur compositions containing between 60 and 100% by weight of insoluble sulfur are used, and the requirements with respect to their thermal stability are continually increasing. By the term "insoluble sulfur composition" as used herein are to be understood sulfur compositions which contain at least 30% by weight of sulfur that is insoluble in carbon disulfide.
Prior proposals for improving the characteristics or properties of insoluble sulfur compositions provided only solutions to one or more but not all of the problems that are referred to hereinbefore. For example, so-called process oils, that is, certain naphthenic or paraffin oils in amounts from 10 to 40% by weight have for a long period been added to insoluble sulfur compositions but produced essentially only a decrease in the amount of dust.
In U.S. Pat. No. 3,706,708 is described a process of admixing alkylphenoxy poly(ethyleneoxy)ethanols with insoluble sulfur used for vulcanizing to improve the dispersibility of the insoluble sulfur in rubber. By supplementary addition of a suitable quantity of oil, powdered products could also be produced. Powdered products of this process had poor flowability characteristics and some of them tended to become charged electrostatically to a greater degree than untreated insoluble sulfur.
Attempts have also been made to convert insoluble sulfur to a more compact form by addition of binders. The production of such molded or shaped bodies is complicated, however, by the concomitant requirement that the molded body completely disintegrate into its component particles when it is mixed with rubber or other plastic substances under pressure and dispersed homogeneously therein. Moreover, the added binders must not impair the stability of the insoluble sulfur and the characteristics of the rubber mixture.
A method is described in the U.S. Pat. No. 3,431,214 for making nondusting, free-flowing, stable sulfur pellets by compacting a mixture of sulfur with from about 2 to about 40% by weight of petrolatum, which is also known as petroleum jelly. Although the pellets thus produced are free-flowing and not dusty, a molding apparatus is required for their production and the pelleting operation also represents a loss of time. Besides that, petrolatum in amounts of up to 40% of the weight of the insoluble sulfur, is not a conventional additive to rubber mixtures and consequently a longer period of mixing is required to produce the desired rubber product.
A process for producing a pelleted sulfur is described in U.S. Pat. No. 3,012,985, in which a pressed pellet is made by extruding a paste formed from 100 parts of insoluble sulfur with between 0.1 and 1.25 parts of rubber latex, between 0.025 and 0.25% of a surfactant, between 0.25 and 1.5 parts of an oil or plasticizer, and sufficient water to form an extrusile paste. The extruded pellets are then dried at a temperature between 50.degree. and 90.degree. C. However the product is not usable because the pellets after drying disintegrate into powder. During the production of the pellets the insoluble sulfur is converted to soluble sulfur, which is undesirable and cannot be prevented because of the alkaline stabilizers which are normally added to latexes to prevent their premature coagulation.